W. Ambach

Solar UV-A and UV-B radiation fluxes at two Alpine stations at different altitudes

SummaryDaily totals of UV-A and UV-B radiation fluxes and global radiation were measured since 1981 at Jungfraujoch (3576 m) a.s.l.) and in Innsbruck (577 m a.s.l.) in their seasonal course. The altitude effect of annual totals yields 19%/1000 m (UV-B), 11%/1000 m (UV-A) and 9%/1000 m (global radiation) with reference to Innsbruck station. The ratio of the daily totals of UV-B/global radiation shows a significant seasonal course with the maximum in summer, whereas the ratio of the daily totals of UV-A/global radiation shows no significant seasonal variation. The biological effective doses of erythema reaction, delayed tanning and immediate tanning by UV-A and UV-B radiant exposure are reported in the seasonal course at Jungfraujoch and in Innsbruck.

Effects of cloudiness on global and diffuse UV irradiance in a high-mountain area

SummaryAt the high-mountain station Jungfraujoch (3576 m a.s.l., Switzerland), measurements of the radiation fluxes were made during 16 periods of six to eight weeks by means of a Robertson—Berger sunburn meter (UVB data), an Eppley UVA radiometer and an Eppley pyranometer. Cloudiness, opacity and altitude of clouds were recorded at 30-minute intervals. A second set of instruments was employed for separate measurement of the diffuse radiation fluxes using shadow bands. The global and diffuse UVA- and UVB radiation fluxes change less with cloudiness than the corresponding total radiation fluxes. When the sun is covered by clouds, the global UVA- and UVB radiation fluxes are also affected less than the global total radiation flux. The roughly equal influence of cloudiness on the UVA- and UVB radiation fluxes suggests that the reduction is influenced more by scattering than by ozone. Also, the share of diffuse irradiance in global irradiance is considerably higher for UVA- and UVB irradiance than for total irradiance. At 50° solar elevation and 0/10 cloudiness, the share is 39% for UVB irradiance, 34% for UVA irradiance and 11% for total irradiance. The increased aerosol turbidity after the eruptions of El Chichon and Pinatubo has caused a significant increase in diffuse total irradiance but has not produced any significant changes in diffuse UVA- and UVB irradiances.

Measuring spectral and spatial variations of UVA and UVB sky radiance

Spectral sky radiance measurements have been obtained with a double monochromator using a 1.5° field of view under varying atmospheric conditions at selected sites in Europe. Spatial variations of the intensity of the sky radiance up to a factor of 10 are observed in the UVA, which decrease to a factor of 2 in the UVB. A distinct minimum of the sky radiance is observed, the location depends on the solar zenith angle but not on wavelength. It is located at scattering angles between 90° and 75° for solar zenith angles between 80° and 26°. A comparison between measurements of the sky radiance and radiative transfer calculations with an aerosol free atmosphere shows a strong dependence of UVB sky radiance on aerosol particles in contrast to UVA sky radiance and possibly a dependence on tropospheric ozone as well.

Experimental investigation of spectral global irradiance measurement errors due to a non ideal cosine response

Due to limitations in the diffusers used for global irradiance measurements, the method usually applied to correct global irradiance measurements is investigated. Based on sky radiance measurements, the assumption of a homogeneous sky radiance distribution generally used to calculate the diffuse part of the global cosine error is accurate to within ±1.5% in the UVB for varying atmospheric and geographic conditions. At longer wavelengths, the assumption of a homogeneous sky radiance distribution should not be used for global cosine corrections since it underestimates the true diffuse cosine error by up to 10% at 500 nm. The accuracy of diffuse irradiance measurements using shading disks is investigated and its dependence on different aerosol amounts is shown. The cosine correction of global irradiance measurements is shown to be insensitive to errors in the determination of the fraction of direct to global irradiance. The uncertainty in the cosine correction of global irradiance measurements is therefore less than ±2% in the UVB.

Total atmospheric ozone determined from spectral measurements of direct solar UV irradiance

With a double monochromator, high resolution spectral measurements of direct solar UV-irradiance were performed in Arosa during February and March, 1993. Total atmospheric ozone amount is determined by fitting model calculations to the measured spectra. The results are compared with the operationally performed measurements of a Dobson and a Brewer spectrometer. The total ozone amount determined from spectral measurements differs from the results of the Dobson instrument by −1.1±0.9% and from those of the Brewer instrument by −0.4±0.7%.

Spectral transmission of the optical media of the human eye with respect to keratitis and cataract formation

The spectral transmissions of cornea, aqueous humour, lens and vitreous humour of human eyes were measured in the range from 250 nm to 800 nm in 1 nm steps using a high resolution double monochromator. Cornea and lens are good cut off filters; 1% transmission was measured in various cornea samples between 291 and 298 nm, and in different lenses between 389 and 414 nm. Aqueous humour and vitreous humour showed high transparency (80%) at⋋ >320 nm. The wavelength range of keratitis effective irradiance was found to be completely within the wavelength range of absorption of the cornea, that of cataract effective irradiance was found to be at the short wavelength end partly outside the wavelength range of absorption of the lens. It may therefore be supposed that the action spectrum of cataract obtained by animal experiments is applicable to the human eye only with certain reservations.

Attenuation of Erythemal Effective Irradiance by Cloudiness at Low and High Altitude in the Alpine Region

Abstract— The transmittance of cloudiness was examined for the daily totals of global erythemal effective irradiance and global total irradiance in Innsbruck (577 m above sea level [a.s.l.], Western Austria) for the periods 1981‐1988 and 1993‐1994 and at Jungfraujoch (3576 m a.s.l., Switzerland) for the period 1981‐1990. The influence of varying cloudiness on the daily totals of global erythemal effective irradiance is considerably greater than the influence of varying ozone. The lowest transmittance for daily totals of global erythemal effective irradiance is 9.4% in Innsbruck (577 m a.s.l.) and 23.5% at Jungfraujoch (3576 m a.s.l.); the median and the 25‐75 percentile range at 10/10 cloudiness are 41.8% (28.9‐56.7%) and 76.8% (63.5‐86.1%), respectively. The greater transmittance of global erythemal effective irradiance at Jungfraujoch originates from smaller thickness of the cloud layer in high mountains than in valleys. Similar transmittances were obtained for the daily totals of global total irradiance (300‐3000 nm) as a function of cloudiness.

SPECTRAL MEASUREMENTS OF GLOBAL AND DIFFUSE SOLAR ULTRAVIOLET‐B RADIANT EXPOSURE AND OZONE VARIATIONS

Since 1985, spectral solar UV‐B radiant exposure has been measured at the High Alpine Research Station Jungfraujoch, Bernese Alps, Switzerland (3576 m). Global (sun and sky) and diffuse (sky) UV‐B radiant exposure on a horizontal plane are measured simultaneously by two identical devices. The detecting system consists of a rotating filter wheel with 8 interference‐filters, centered at every 5 nm between 290 and 325 nm, and a halfbandwidth of ±3 nm. The special advantage of this detector is its waterproofness, allowing operation under all weather conditions. The ratio of the intensities at 305 and 315 nm is being analyzed with regard to ozone variations; for example, the ratio is reduced by a factor of 2 because of an increase in ozone content from 250 to 350 Dobson units. The relation between ozone variations and measured spectral UV‐B intensity is confirmed applying Greens radiation model.

OZONE AND ULTRAVIOLET-B IRRADIANCES: EXPERIMENTAL DETERMINATION OF THE RADIATION AMPLIFICATION FACTOR

Abstract— During the period 1981–1993, measurements of solar UV irradiances were made at the High‐Alpine Research Station Jungfraujoch (Switzerland, 3576 m a.s.l.) to determine the radiation amplification factor (RAF) for the Robertson‐Berger sunburn meter and for the narrow‐band wavelength ranges of the Eppley filter spectrometer. The Robertson‐Berger sunburn meter model 500 showed a RAF of 1.07 ± 0.15 at solar elevations between 20° and 60°. The RAF for human erythema is 1.1 in comparison. Therefore the Robertson‐Berger sunburn meter is suitable to measure the influence of total atmospheric ozone variations on the effective erythemal irradiance. In the narrow‐band wavelength ranges of the Eppley filter spectrometer the RAF increases greatly at shorter wavelengths with RAF of 1.06, 1.40 and 2.35 for the optical centers 315.1 nm, 311.1 nm and 305.2 nm, evaluated at 30° solar elevation. In order to minimize perturbations by aerosol optical depth and albedo in the evaluation of the RAF the ratios of UV irradiances to total irradiances were evaluated.

ERYTHEMAL UV-B IRRADIANCE (ROBERTSON-BERGER SUNBURN METER DATA) UNDER OZONE DEFICIENCIES IN WINTER/SPRING 1993

Abstract –Robertson‐Berger sunburn meter data recorded from August 1992 to May 1993 (Innsbruck, 577 m above sea level, Austria) were compared with measurements taken in previous years. In February and March 1993 instantaneous values on clear days were found to be higher by up to 37% than those registered in previous years (1986–1988). However, in winter/spring 1993 daily totals for all days, averaged over 7 days, were within the range of standard deviation of the previous years (1981–1988). Therefore the cumulative UV‐B exposure of the ecosystem is within the general range of variability even under the record low ozone values in winter/spring 1993, while single peak doses were enhanced significantly.